There has been significant interest in developing high resolution, non-contact optical measurement devices This interest is particularly acute in the semiconductor manufacturing industry where process steps are performed on a very small scale.
One example of a high resolution optical measurement device is described in U.S. Pat. No. 4,999,014, issued Mar. 12, 1991 assigned to the same assignee as the subject invention and incorporated herein by reference. This device is designed for measuring the thickness of a thin film layer on a substrate.
FIG. 1 is an illustration of the relevant portions of the above referenced device 10. Device 10 includes a laser 12 for generating a probe beam 14. A beam splitter 16 is used to redirect the beam down through a microscope objective lens 20 having a high numerical aperture. The lens 20 focuses the probe beam to a spot on the surface of the sample 18. The diameter of the spot is on the order of one micron. A stage 22 is provided to scan the sample with respect to the focused probe beam.
A photodetector 26 is provided to measure the probe beam after it has reflected off the surface of the sample. A relay lens 28 is provided to expand and image the beam on the detector. Detector 26 includes an array of individual detector elements capable of measuring the areal intensity of various rays as a function of their position within the reflected probe beam. The position of the rays within the beam correspond to specific angles of incidence with respect to the surface of the sample. A processor (not shown) derives the thickness of the thin film layer based on the angular dependent intensity measurements.
In the preferred embodiment, a second photodetector 34 is provided and arranged in a similar optical position as detector 26. A lens 36 is provided to image the beam on the detector 34. This second detector 34 is underfilled and configured to measure the full power of the reflected probe beam. The output from detector 34 is used to enhance the sensitivity of the evaluation.
In the arrangement discussed above, it would be expected that the light reaching either of the detectors 26 or 34 would be limited to that which has been reflected off the surface of the sample from within the focused spot. In practice, it has been found that some small amount of the light falling on the detectors has been reflected off the sample from areas outside of the focused spot. This light consists of portions of the probe beam that have been scattered or deflected out of the primary beam path. Such scattering effects can be produced by particulates in the air or on the lens. This effect can also be the result of imperfections in the lens or even in the beam itself. Experiments have shown that light energy can be measured by the detectors 26 and 34 from areas as far as 20 to 30 microns away from the edge of the focused spot. Since the device is intended to evaluate sample parameters only in the small one micron region, light reflected from areas outside the focused spot can improperly skew the measurement results.
While the amount of scattered light detected is relatively low, it is nonetheless significant. For example, changes on the order of 1 in 10,000 must be distinguished when measuring the full power of the reflected beam. Thus, even minor amounts of scattered light reaching the detector can adversely affect the analysis of the sample. Therefore, it would be desirable to provide an approach which can reduce the effects of light which has been reflected from areas outside of the focused spot.
The subject invention is not limited to the apparatus described in the above cited application but might be used in any optical device where high resolution and high sensitivity are required. One example of another type of device where the subject invention could be utilized is described in U.S. Pat. No. 5,042,951, issued Aug. 27, 1991 assigned to the same assignee as the subject invention and incorporated herein by reference.
The apparatus described in the latter application is an ellipsometric device which has a configuration similar in many respects to the device shown in FIG. 1 herein. The principle difference is that the detector system is arranged to analyze the change in polarization state of various rays within the probe beam as a result of its reflection off the sample surface.
There are a number of approaches found in the prior art for detecting the change in the polarization state of a probe beam. In its basic form, the apparatus will be provided with a polarizing section 40 (shown in phantom line) and an analyzing section 42 (also shown in phantom line). The polarizing and analyzing sections can include polarizing elements which can be rotated about the propagation axis of the beam. By knowing the relative azimuthal positions of these rotatable elements in conjunction with the intensity measurement of detector 26, the change in polarization state of the beam can be determined. The latter analysis will also be adversely affected by detected light that has been reflected off the sample surface from areas outside of the focused probe beam spot.
FIG. 1 illustrates one additional photodetector 46. Photodetector 46 is arranged to a measure a small portion of the probe beam that is transmitted through beam splitter 16. This portion of the light has not passed through lens 20 nor has it been reflected from the sample. The output of detector 46 is intended to monitor fluctuations in the output power of laser 12. The signal generated by detector 46 is used to normalize the output signals from detectors 26 and 34. In order to obtain an accurate normalization signal, it is necessary to insure that the portion of the probe beam striking detector 46 is the same as that which is passed through the aperture of the lens 18 to the sample. An approach for achieving that goal is also discussed below.